The present invention relates to apparatus for temporary storage of a series of sheets, particularly for temporary storage of a stream of partially overlapping paper sheets. Still more particularly, the invention relates to improvements in apparatus for temporary storage of a preferably scalloped stream of paper sheets between the convolutions of a band which is wound onto a core or another suitable rotor.
It is well known to store a scalloped stream of imprinted sheets on the core (first rotor) of a reel by resorting to an elongated band one end portion of which is secured to such core and the other end portion of which is affixed to a second rotor (e.g., a second core). The sheets are delivered onto the band slightly ahead of or at the locus where the band is being wound onto the first rotor, and such sheets form in their entirety a long helix whose convolutions alternate with the convolutions of the band on the first rotor. A prime mover is provided to drive the first rotor in a direction to collect the band, and a brake serves to oppose the movement of the band in a direction from the second toward the first rotor so that the band is maintained under tension, i.e., the brake causes the development of a braking moment acting upon the roll of convoluted band and stream of sheets on the first rotor. Reference may be had, for example, to German Pat. No. 25 44 135 and to German Offenlegungsschrift No. 31 23 888.
The apparatus of German patent No. 25 44 135 comprises means for delivering successive sheets of a scalloped stream of sheets onto the upper side of that portion of the band which is about to be convoluted onto the first rotor so that the sheets are confined between neighboring convolutions of the band. The brake acts upon the band between the two rotors and the band confines successive sheets of the stream between that portion thereof which is immediately adjacent to the growing roll on the first rotor and the outermost convolution of the band on such rotor. A drawback of the patented apparatus is that its first rotor can store only a limited number of freshly imprinted sheets. The reason is that the pressure which the convoluted material applies upon the innermost convolutions of the stored stream of sheets increases with the addition of each fresh convolution of the band. This is due to the fact that the braking device acts upon the band with an unchanging force so that the magnitude of compressive stress acting upon the convolutions of the stored stream increases with each revolution of the first rotor in a direction to gather the band. As the inner convolutions of the stored stream undergo a progressively increasing compressing action, the tensional stress upon the neighboring convolutions of the band decreases with the result that the adjacent sheets can shift relative to each other and relative to the neighboring convolutions of the band. This can adversely influence the quality of the product which is assembled from such sheets after the stream is removed from storage. Furthermore, any relaxation of tension upon the inner convolutions of the band on the first rotor can entail at least some fulling of sheets with attendant drawbacks, such as shifting of the two halves of each sheet which has been folded prior to introduction into storage.
The mode of operation of the apparatus which is disclosed in the aforementioned Offenlegungsschrift is practically identical with that of the apparatus of the aforediscussed German patent. Thus, the apparatus of the Offenlegungsschrift also employs a brake which applies to the band a braking force of unchanging magnitude. Therefore, the inner windings of the stored portion of a stream of paper sheets are subjected to a compressive stress which increases with increasing diameter of the convoluted roll of sheets on the first rotor. Furthermore, the magnitude of torque which must be applied to the first rotor increases proportionally with increasing diameter of the convoluted material. This, combined with the aforediscussed progressively increasing compression of inner convolutions of the stored portion of the stream imparts to the convoluted band a tendency to hang furthermore, each inner convolution of the band on the first rotor moves circumferentially of the first rotor with reference to the next (outer) convolution. This, entails a pronounced shifting of one-half of each folded sheet between the neighboring convolutions of the band relative to the other half (note FIG. 4) so that the position of the fold line between the two halves of each such sheet is changed with attendant problems when the sheets are removed from storage and introduced into a gathering or other sheet processing machine. The problem is aggravated if the band is made of or is coated with an adhesive material. Thus, if one side of the band tends to adhere to the sheets, the sheets (or at least certain sheets) cannot be readily or predictably removed from storage. This will be appreciated by bearing in mind that, if the sheets are provided with multicolored printed matter, certain coloring agent or agents will adhere to the band with a greater force than the other coloring agent or agents. Furthermore, and even if the adherence of sheets to an adhesive band does not present serious problems, the band is likely to cause the development of streaks which affect the appearance of freshly imprinted sheets as well as of the books, brochures or other products which embody such sheets.